The present invention relates to fuel channels for nuclear reactors. More particularly, the invention relates to annulus spacers for use in fuel channels having concentric tubes (e.g., a calandria tube with an internal pressure tube) of a nuclear reactor, an example of which is the CANDU (“CANada Deuterium Uranium”) reactor. By way of example only, the CANDU reactor is a heavy water, or light water cooled and heavy-water moderated fission reactor capable of using fuels composed of natural uranium, other low-enrichment uranium, recycled uranium, mixed oxides, fissile and fertile actinides, and combinations thereof.
Annulus spacers (AS), often provided as garter springs (GS), are used in CANDU reactors to maintain an annular gap between two tubes of a fuel channel assembly, such as an inner pressure tube (PT) and an outer calandria tube (CT) as mentioned above. The PTs are located inside the reactor CTs that insulate the PTs from the heavy water moderator in the calandria. The annular gap between the PTs and CTs is typically filled with an annulus gas. In some cases, four annulus spacers are used per fuel channel assembly, each at a specified axial position along the length of the fuel channel. It is important that the spacers are in their correct positions, as incorrect positioning may lead to contact between the hot PT and cooler CT. Such contact is unacceptable.
In some embodiments, a fuel channel assembly consists of a PT, two end fittings and associated hardware, wherein the PT is connected to the two end fittings by a mechanical roll-expanded joint. A bellows assembly rolled into the fuelling machine side tubesheet and welded to the bellows attachment ring can be used to seal the annulus at both ends. Therefore, in some fuel channel assembly embodiments, there is no direct access to the annular space between the PTs and CTs.
One known type of annulus spacer is a close-coiled helical spring. For example, such a spring can have a 4.83 mm (0.190 inch) outside diameter, can be formed into a torus using a Zircaloy-2 girdle wire, and can be formed from Zirconium alloy wire of square cross-section (e.g., 1.02×1.02 mm (0.040×0.040 inch)). The spacers can prevent direct contact between the PTs and CTs, which would be undesirable because of the increased susceptibility to blister formation as local hydrogen concentration increases due to deuterium ingress. In some embodiments, there are four spacers in each channel assembly spaced approximately 1.02 m (40 inches) apart and located in a manner offset towards the outlet end of the fuel channel assembly. The position of each of the annulus spacers is important to ensure that they meet a variety of functional, performance, safety, environmental and inter-facing system requirements.
Some annulus spacers are loose-fitting spacers provided with a garter spring and a girdle wire held within an annular cavity formed by the coiled wire of the garter spring. The girdle wire can enable a position of the annulus spacer along the fuel channel to be detected using eddy current testing (ECT) techniques (i.e., based upon the fact that the girdle wire can be made from a material that forms a loop of continuous conductivity). Other annulus spacers are tight fitting, and can have a spring tension which draws them tight onto the outside surface of the PT.
In many cases, eddy current technology cannot positively locate tight fitting spacers because the design of tight fitting spacers does not include a welded girdle wire. Other challenges with loose- and tight-fitting spacers exist, as do challenges to identifying the locations of such spacers along the axial length of fuel channel assemblies.